The present invention relates to a method and a device for optical pressure measurement of a gas in a closed container.
For the measurement of the pressure in beverage bottles, methods based on repeated volume expansion are known (AT 409 673, GB 2,373,584, U.S. Pat. No. 6,874,351). These methods permit only random sampling measurements, because the beverage has to be filled into an extra measurement chamber. Even the pressure sensors placed into the bottles, known from DE 10 2007 027 708 A1 for instance, do not permit online/inline measurement. From DE 10 2006 001 902 B4, DE 10 2006 030 296 A1 and JP 00 2006 030 161 AA, methods in which the absorption line of the enclosed gas is measured with the aid of a laser are known for optical pressure measurement in the closed container. For this purpose, it must be ensured that the laser emits a longitudinal mode only, which is spectrally tunable in a reproducible manner besides to this. This is permitted exclusively by expensive lasers. Moreover, very precise control of the operating current and the operating temperature of the laser is necessary for this, which further increases the expenditure. The exact measurement of an absorption line takes so much time that these methods are not suitable for automatization. Because absorption lines widen up so strongly at high pressures that they melt together with the neighbouring lines, as will be outlined in more detail below, the lines will become indistinguishable upon increasing pressure, and the precision of measurement decreases. From on a certain pressure, the lines will become indistinguishable. Thus, the method has only a limited measurement range and is intended for low pressures.
From EP 1 450 139, a method is known to measure the entire flow of air masses through a combustion engine. For this purpose, the concentration of carbon dioxide is determined by detecting the infrared spectrum. For analysis, the measured carbon dioxide absorption spectrum is compared with a multitude of mathematically simulated carbon dioxide absorption spectra in order to find a coincidence. The mathematically simulated carbon dioxide absorption spectra correspond to different temperatures and different concentrations of the carbon dioxide.
From US 2005/0022603 A1, a method is known to measure the pressure in a closed container. In this method, the wavelength of a laser is periodically modulated in a wavelength range which comprises at least one absorption line of the gas to be examined. The electric output signals developing through the periodically modulated wavelength are subsequently subject to frequency filtering by a bandpass filter or a filter having a cutoff frequency. The filtered signals are then analysed in view of the pressure of the gas.